dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Wed, Dec 17, 2008 at 09:18:42AM +0100, Martin Sandve Alnæs wrote:
> On Wed, Dec 17, 2008 at 8:44 AM, Anders Logg <logg@xxxxxxxxx> wrote:
> > On Wed, Dec 17, 2008 at 12:15:38AM +0000, Garth N. Wells wrote:
> >> I'm running into some problems with the new Function design when trying
> >> to update a solver. I'll try to sketch the issue as simply as possible.
> >>
> >>      Function v;
> >>      Function u, p;
> >>
> >>      // First form for U = (u, p)
> >>      FirstBilinearForm a_1(V1, V1);
> >>      FirstLinearForm   L_1(V1);
> >>      LinearPDE pde_first(a_1, L_1);
> >>
> >>      // Second form which depends on u
> >>      SecondBilinearForm a_2(V2, V2);
> >>      SecondLinearForm   L_2(V2, u);
> >>      LinearPDE pde_second(a_2, L_2);
> >>
> >>      Function U;
> >>      for(uint i = 0; i < 10; ++i)
> >>      {
> >>        pde_first.solve(U);
> >>
> >>        // This step breaks down because FunctionSpaces don't match
> >>        u = U[0];
> >>        p = U[1];
> >>
> >>        pde_second.solve(v);
> >>      }
> >>
> >> The problem is in assigning Functions since we now check the FunctionSpace.
> >
> > Which check is it that fails?
> >
> >> To get around this, I tried
> >>
> >>      FunctionSpace V(mesh);
> >>      Function U(V);
> >>      Function u = U[0];
> >>      Function p = U[1];
> >>
> >> which throws an exception because U does not have a vector.
> >
> > What if we remove the check in operator[] and then in the constructor
> > that gets a SubFunction we rely on v.v.vector() which (after Martin's
> > fix from yesterday) will always return a vector with the dofs
> > (possibly interpolated if there were no dofs before).
> 
> Depends...
> 
> Function u;
> Vector & v = u.vector(); // error, no function space, ok
> 
> MyFunction u(V);
> Vector & v = u.vector(); // calls interpolate, using MyFunction::eval
> 
> Function u(V);
> Vector & v = u.vector(); // What happens here?

I looked more closely at this, and as far as I can tell, your two last
examples will result in the same thing: a zero vector will be created:

if (!_vector)
{
  init();
  interpolate(*_vector, *_function_space);
}

The call to init() first creates a zero vector. Then the function will
be interpolated and the coefficients placed in the vector. This
ultimately results in a call to

  Function::interpolate(double* coefficients,
                        const FunctionSpace& V,
                        const ufc::cell& ufc_cell,
                        int local_facet) const

This function check if there is a vector of dofs (which there now is)
and in that case picks the dofs from the vector (without calling
evaluate_dof or eval).

> Also, I had another function space error yesterday.
> Trying to interpolate a user function into a discrete
> function space fails because the user function doesn't
> have a function space, but that shouldn't be necessary.
> (btw, all the "interpolate" variants makes it difficult to
> discuss and remember exact signatures etc...)

Should we rename some of them?

-- 
Anders

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